pointer-gestures: add a release request

This allows clients to destroy a gesture object before they disconnect.

The request isn't named "destroy", as this would conflict with
wayland-scanner's auto-generated destructor (which just destroys the
client-side object without sending any request).

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>